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River
Filtration in water Treatment Water
Definition of Sedimentation: Screen
Filtration is a solid –liquid separation process in which the
liquid passes through a porous medium to remove as
much fine suspended solids as possible Pre‐Sedimentation
Sedimentation
Coagulation Flocculation
Locations of filtration tanks in water treatment:
Filtration tanks are used in all types of water treatment
plants except for disinfection treatment plants.
Filtration Disinfection Storage Distribution
Figure 5.1: Filtration Treatment Plant (River
Water)
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Screen
Surface
water Hard
Sedimentation Water
Coagulation Flocculation Rapid Mixing Flocculation Sedimentation Recarbonation
Filtration Disinfection Storage Distribution
Filtration Disinfection Storage Distribution
Figure 5.2 : Filtration Treatment Plant Figure 5.3: Softening Treatment Plant Single stage
softening
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Need for filtration:
Settling is not sufficient to remove all particles and
Ground flocs from water.
Water Aeration Filtration
well
Filtration Needed for fine particles not removed by
sedimentation.
Filters can also capture Giardia cysts, viruses, and
asbestos fibers
Storage Distribution
Disinfection Typical overflow qualities from sedimentation tanks
range from 1 to 10 NTU.
Filtration, usually rapid sand filtration, is then
Aeration treatment Plant (Iron and Manganese Removal Plant) employed for further “polishing”, i.e. to get the
turbidity to lower than 1.0 NTU (as required by
standard).
Rapid sand filtration after prior sedimentation is the
most common configuration worldwide
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FILTRATION CLASSIFICATION OF FILTERS
1) According to type of granular medium used
single medium (sand or anthracite )
DEEP BED FILTRATION CAKE FILTRATION
dual media (anthracite and sand )
(depth filtration)
Particle removal occurs multi media (anthracite, sand, garnet)
Solids are removed largely at the surface of
within a bed of porous the media through
material formation of a filter cake
Dual media filters better
longer filtration run
e.g. Rapid granular bed e.g. Pre-coat filtration
filters (diatomite ,
diatomaceous earth) Available pore volume is maximum at the top of
filter and gradually decreases to a minimum at
Slow sand filters
the bottom of filter
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5. Filtration in water Treatment Filter media properties
The filter media is characterized by two main
5.9 Filter media properties parameters: the effective size and the uniformity
coefficient.
Effective size of the filter media
The effective size of the media is the diameter
that 10% of the filter media is less than it size and
is denoted as d10.
Uniformity coefficient of the filter media
d 60
U
d10
U =Uniformity coefficient
d60 = sieve size that passes 60% by weight
d10 = sieve size that passes 10% by weight
‐ d60 and d10 are found by sieve analysis of the
media to be used in the filter.
filter media grain distribution ‐Another important sieve size is d90 that is used
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CLASSIFICATION OF FILTERS CLASSIFICATION OF FILTERS
2) According to flow throughmedium 3) According to rate of filtration
G r a v i t y filters R a p i d sand filters
are open to the atmosphere
Flow through the medium is achieved by gravity S l o w sand filters
P r e s u r e filters 3) According to filter flow control scheme
Filter medium is contained in pressure vessel C o n s t a n t rate
Water is delivered to the vessel under pressure D e c l i n i n g rate
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Slow Sand Filtration Slow Sand Filtration
Removal Mechanism:
very low filtration rate ( 0.1 – 0.4 m/hr)
SAND particle size smaller than that for rapid sand filters
Effective size of sand used = 0.1 – 0.3 mm all of the suspended materials being removed at the filter surface
Uniformity coeff. = 2-3
Thickness of the bed = 1 – 1.5 m
a mat of biological organisms is allowed to develop at the water –sand interface
Supporting gravel layer = = 0.3 –0.5 m (prevents the penetration of fine sand particles into lower which aided in the filtration process (Schmutzdecke Layer )
layers) Accumulation of Schmutzdecke layer ranges between 6 hr to 30 hr
large space requirement dayso Optimum operation is not obtained till Schmutzdecke layer is formed
underdrain normally perforated pipes placed within the lower portion of the supporting
gravel layer
conventionaly operated at a constant rate 98 99
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Slow Sand Filtration Slow Sand Filtration
Principal Uses:
Cleaning of bed:
removal of organic matter and pathogenic organisms from raw waters of relatively low
sand bed is cleaned by removing schmutzdecke along with a small amount of sanddepth,
turbidity ( < 50 NTU)
an operation known as scraping (either by hand or mechanically)
In Schmutzdecke layer - biological treatment Scraping of the small amount of media decrease of sand bed
reduction of total bacteria count by a factor of 103 to 104
good for complete removal of Giardia Cysts. When the bed reaches a minimum thickness of 0.5 – 0.8, resanding of
for low turbidity surface waters c h e a p e s t , simplest filters
Operation:
During operation, the level of water above the filter surface will gradually increase as the
upper layers of the sand becomeplugged.
When the water level above the filter medium has increased to 1.25 to 2 m, the filters are
cleaned 16
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Pressure Filters Pressure Filters
Advantages & Disadvantages
The filter medium is contained in a steel pressure vessel
influent is under pressure higher filtration rate
high terminal head loss
may be a cylindrical tank with vertical axis
may be a horizontal axis cylindrical tank
tend to be used in small water systems ( e.g industrial applications) water enters and leaves the filter under pressure
no negative pressure can ever exist in filter
operating principals are identical with those of gravity filters
filter medium is in a closed vessel (i.e, it is not conveniently visible)
water to be filtered enters the filter under pressure and leaves at slightly reduced
proper backwashing is difficult
pressure becuse of the headloss encountered in the filter medium, underdrain and piping
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Rapid sand filters
Geometry and components of Rapid Sand Filter:
Rapid sand filters are always rectangular tanks.
Main components of Rapid sand filter are:
1. A concrete tank
2. Filter media
3. Under-drain system
4. Backwash system: pressurized water and air lines
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Rapid sand filter components
Rapid sand filter components : with gravel and
perforated pipes under drain system
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Rapid sand filter components : with gravel and
Rapid sand filter components : with gravel and perforated pipes under drain system
perforated pipes under drain system 12 24
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Rapid sand filter perforated
slab and nozzle under‐drain
system
Rapid sand filter components: with nozzle under‐ drain system
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Operation of Rapid Sand Filter:
There are two modes of operation of Rapid sand filter
Filtration mode
Backwashing mode
Nozzle used in Rapid sand filter
under‐drain system
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Backwash mode:
Sand is backwashed when
Filtration mode: – It becomes clogged, or
– Turbidity of filtered water gets too high
• Water flows downward through a bed
of sand and gravel – Maximum time limit
• Particles are captured on and between
sand grains
• Filtered water is collected in the
under-drain, sent to disinfection
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Head loss and effluent turbidity increase with time during filtration
Figure 5.14 : Head loss in rapid sand filter
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Backwash
• During backwash, water is pumped The methods used for backwashing granular medium filter beds:
upwards through the sand bed • water backwash with full fluidization
• Sand becomes “fluidized”, and particles
are flushed from the sand • surface wash plus fluidized bed
• Dirty backwash water is pumped into a backwash
settling pond, and either
– Recycled back into plant, or • air scour-assisted backwash
– Disposed
• Backwashing can consume 1% to 5% of a
plant’s production
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